Apparatus for dissolving nuclear fuel

ABSTRACT

A nuclear fuel dissolution apparatus which comprises a perforated sloping ramp contained within a process chamber for containing solvent for the fuel, a pulsation member which in use creates pulses in solvent in the process chamber, the perforations being designed to direct pulses of solvent along and up the ramp, and a discharge point for fuel hulls disposed at an upper region of the ramp is characterised in that the ramp is made out of flat blades and the perforations of the ramp comprise inclined slits formed between the blades.

The present invention relates to the reprocessing of irradiated nuclearfuel and particularly the dissolution of nuclear fuel pins.

Nuclear fuel pins consist of pellets of fissile material, e.g. UO₂,contained in a cladding which is normally a zirconium alloy sold underthe trade Zircaloy. A cluster of pins form a fuel assembly.

Commercial reprocessing of irradiated (spent) nuclear fuel uses thePurex process, which involves chopping up the pins of an assembly priorto dissolution of the fissile material in nitric acid. The pins must bechopped up to expose the pellets to nitric acid because the bulkzirconium alloy is resistant to attack by nitric acid, as is an oxideskin which irradiated zirconium alloy possesses. After the fuel has beendissolved the empty pieces of cladding (hulls) and other residues of thestructure of the fuel assembly are encapsulated and disposed of.

Commercial reprocessing plants contain dissolvers based on one of twoprinciples: batch operation or continuous operation using an apparatuswith rotating mechanical parts.

In a typical batch dissolver the fuel is sheared into a large basketwhich is immersed in the dissolver vessel. After the fuel has beenleached, the basket is removed from the dissolver using a crane and thentipped to transfer the hulls and other debris to the encapsulationplant. This system requires extensive mechanical handling of thedissolver basket which is heavy and needs a large amount of maintainableequipment in the shielded dissolver cell.

Continuous dissolvers shear the fuel into the segments of a wheel whichis rotated inside a large vessel. The existence of rotating parts is adisadvantage and limits the geometry that can be adopted to a very largeslab tank if the equipment is ever to be safe.

U.S. Pat. No. 4,246,238 discloses a batch action apparatus comprising acontainer in which is placed a basket filled with pieces of fuel pins,and to which pipe connections are fitted to feed in and remove solutionsand gas. A disadvantage of the apparatus is the complexity of thesealing arrangement when the fuel pin pieces are loaded into the basketand when baskets are discharged from the apparatus, as well as thedifficulty of containing radioactivity. In addition, use of suchapparatuses would considerably complicate process automation andincrease the volumes of radioactive solutions.

U.S. Pat. No. 4,230,675 discloses an apparatus which is used to makecountercurrent contact between fuel pin pieces and leaching solution.The apparatus comprises an elongate cylindrical drum which rotatesaround its longitudinal axis and is divided by transverse partitionsinto a chamber for feeding in fuel pin pieces and removing solutionsituated at one end of the drum, and a chamber for exit of the claddingpieces and delivery of solution situated at the other end of the drum.Between these chambers there are several other chambers in whichleaching of the fuel from the chopped cladding takes place. In eachchamber there are elements which provide for movement of the fuel pinpieces on rotation of the drum. A disadvantage of the apparatus is thecomplexity of its design, and the presence of moving units and of unitssubject to intensive abrasive action caused by the fuel pin pieces. Anapparatus of this design would therefore have a low level of reliabilityand require regular servicing and maintenance under conditions of highlevels of radioactive contamination.

Moving away now from nuclear reprocessing, USSR author's certificate No.764698 discloses a mass-exchange device for solid phase treatment(primarily for the treatment of wood chips) which comprises acylindrical casing inside which is a perforated helical ramp with ahelical surface gradient of between 4 and 30 degrees. The perforationsare in the form of apertures screened above and below by angled plates.Some of the apertures are fitted with nozzles protruding above thesurface of the ramp. In a bottom part of the apparatus, which isseparated from the annular container by a grid, there is a pulsegenerator. In use, solid particles are charged onto the ramp at itsbottom end. Pulsed fluid from the pulse generator passes upwards throughthe apertures in the ramp and raises the solid particles above the ramp,creating a pulsing suspended layer in which the particles are processedby the fluid. The solid particles are moved upwards and discharged atthe top of the ramp.

The apparatus of USSR author's certificate No. 764698 could not be usedwith chopped nuclear fuel pins and is incompatible with a nuclearreprocessing plant. Amongst other things, chopped fuel pins have adiameter of between 8 and 20 mm, a length of between 25 and 100 mm and aweight of up to 70 g and would not be moved up the ramp of the apparatusas described.

A pulsation device which is designed for dissolving nuclear fuel pins isknown from EP-A-358354 which comprises a V-shaped duct connected to apulsation chamber containing nitric acid. Pieces of chopped fuel pin areloaded into the free end of one arm of the V-shaped duct. Compressed airin the pulsation chamber maintains nitric acid in the duct at a levelwhich immerses most of the chopped pieces. The air pressure in thepulsation chamber is periodically released, resulting in most of thenitric acid in the duct leaving it. Compressed air is then again appliedto the nitric acid in the pulsation chamber, causing acid to pulse intothe duct such that leached hulls in the outlet arm of the duct arepushed upward to a discharge duct through which they leave the V-shapedduct.

An advantage of this apparatus is the simplicity of its design and theabsence of moving parts. A disadvantage of the apparatus is the smallvolume of its process chamber and the low output of a single unit, withpieces of fuel pin spending a long time in the apparatus, as isessential for dissolution of the fuel. This is because the diameter ofthe V-shaped duct is restricted for criticality reasons, and the depthof the layer of fuel pin pieces in the duct is limited by therequirement that the pieces must move along steadily in a curving duct.To ensure that the required time is spent by the fuel pin pieces in thesolution, therefore, a cascade of apparatuses of this type installed oneafter the other in sequence would have to be used. This wouldconsiderably complicate the apparatus layout, reduce the reliability ofthe equipment and also substantially increase the dimensions of theprocess areas.

The problem on which a first aspect of the invention is based,therefore, is to provide apparatus for use in the dissolution of nuclearfuel pins in which the apparatus can reliably be used to dissolve fuelcontained in chopped cladding and to discharge hulls from the dissolveras well as require little maintenance and servicing, and which would notrequire a complicated apparatus layout or an excessive process area inthe reprocessing plant.

The present invention provides a nuclear fuel dissolution apparatuscomprising a perforated sloping ramp contained within a process chamberfor containing solvent for the fuel and a pulsation member which in usecreates pulses in solvent in the process chamber, the perforations beingdesigned to direct pulses of solvent along and up the ramp, and a methodcomprising loading solvent into the process chamber, loading fuel pinpieces onto a lower region of the ramp and creating solvent pulses totransport the fuel pin pieces up the ramp to a discharge point where thecladding hulls are discharged from the ramp.

In preferred apparatus the ramp is spiral; the gradient of the spiral ispreferably between 1 and 30 degrees and more usually between 1 and 20degrees. Preferably, the process chamber is annular in cross section.

The gradient of the spiral in an upper zone of the process chamber maybe greater than in a lower zone.

It is most preferred that the ramp is made out of flat blades, in whichcase the perforations of the ramp comprise inclined slits formed betweenthe blades.

Preferably the angle between the plane of the blades and the horizontalis between 10 and 60 degrees.

In practice, the apparatus must be designed to have a so-called“eversafe geometry”, that is, to avoid a critical mass of materialcollecting which allows a self-sustaining fission reaction. For thisreason, the apparatus will in practice be designed to control the totalamount of fissile material which can accumulate in any one place and/orthe geometry in which such accumulated fissile material is held. It ispreferred for criticality reasons for the fissile material (chopped fuelpins) to be in elongate form rather than spherical. In a particularversion of the method, each pair of adjacent blades makes a slit havinga length (the dimension in the radial direction in the case of a spiralramp in an annular chamber) of no more than 10 times the diameter of afuel pin; such a design helps avoid an excessive accumulation of choppedfuel pins in the slit. More preferably, the slit length is between 5 and10 times the diameter of a fuel pin.

In some embodiments the blades are made in the form of a trapezium andare fastened by the smaller end to a central blade support within theprocess chamber.

Preferred apparatus involves one or both of the features that theaverage width of the blades (extent of slit channel) is between 3 and 5times the distances between them and that the distance between theplates at the outside wall of the container is 0.4 to 0.8 times the fuelpin diameter.

Preferably, the pulsation member comprises a pulsation chamber locatedcoaxially within the process chamber. Normally, a neutron absorber isarranged between the pulsation chamber and an inside wall of the annularcontainer.

In a particular version of the invention, the pulsation chamber, usuallymade in the shape of a cylindrical container, is located coaxiallywithin the process chamber (which is usually annular) and communicateswith a lower part of the process chamber by a an upwardly and outwardlydirected duct (usually an annular slit). In preferred classes of theinvention, a neutron absorber is situated between the pulsation chamberand an inside wall of the annular container.

The invention in another aspect provides an apparatus for the treatmentof solid articles by liquid, comprising a container having an outer sidewall of circular cross section, a spiral ramp located in the container,and a pulsator communicating with a lower part of the container, andalso pipe connections for feeding in and removing pieces of fuel pin,solution and gas, characterised in that the ramp is made up of flatblades placed one after another along the spiral and forming between oneanother inclined slit nozzles. Such apparatus is particularly suitablefor the chemical treatment of solid phase articles larger or heavierthan the wood chips with which is concerned the prior art spiral pulsedfluid apparatus.

Usually, the process chamber of either aspect has an inner side wall aswell as the inevitable outer side wall. The ramp in such chambersnormally extends between the inner and outer side walls of the processchamber.

The present invention also provides a method of dissolving fuel inchopped nuclear fuel pins in an apparatus comprising a perforatedsloping ramp contained within a process chamber for containing solventfor the fuel and a pulsation member which in use creates pulses insolvent in the process chamber, the perforations being designed todirect pulses of solvent along and up the ramp, the method comprisingloading solvent into the process chamber, loading fuel pin pieces onto alower region of the ramp and creating solvent pulses to transport thefuel pin pieces up the ramp to a discharge point where the claddinghulls are discharged from the ramp. The method may be performed in thereprocessing of nuclear fuel, the method further including reprocessingthe dissolved fuel to form a fissile material optionally in the form ofa fuel pellet, a fuel pin or a fuel assembly.

The present invention is further described below by way of example onlywith reference to apparatus for dissolving the spent fuel of choppedfuel pins. It will be understood, however, that the invention may beapplied to the fluid treatment of solids other than fuel pin pieces. Theapparatus is illustrated non-limitatively by the accompanying drawings,in which:

FIG. 1 gives a general view of the apparatus for dissolution showing across-section and the appearance of the ramp made up of blades fittedone after the other along the spiral;

FIG. 2 shows a cross-section of the apparatus along line A—A of FIG. 1;

FIG. 3 is an enlarged view of fragment A in FIG. 1 of the internalcylindrical shell of the annular container of the apparatus with rampblades fitted on it;

FIG. 4 shows fragment B of the ramp from FIG. 1;

FIG. 5 shows fragment C of the ramp from FIG. 2; and

FIG. 6 shows the shape of the blades in the form of a rectangular (a)and isosceles (b) trapezium.

FIG. 1, therefore, illustrates a nuclear fuel dissolution apparatuswhich comprises a perforated sloping ramp (19) contained within aprocess chamber (1) for containing solvent for the fuel, a pulsationmember (9) which in use creates pulses in solvent in the process chamber(1), the perforations (7) being designed to direct pulses of solventalong and up the ramp (19), and a discharge point (18) for fuel hullsdisposed at an upper region of the ramp.

More particularly, the apparatus shown is for leaching of fuel in fuelpin pieces and comprises an annular reaction container (1), formed byinternal (2) and external (3) cylindrical shells, and also a drain (4)and a cover (5). In the annular container (1) are blades (6) fitted oneafter another along a spiral between the internal (2) and external (3)walls of the container at a distance not exceeding the diameter of thefuel pin. (It will be appreciated that an apparatus of the inventioncannot be used to treat articles able to fall between the blades (6)).

The blades form a spiral ramp for upwards movement of pieces of fuel pinupon pulsation of solvent (nitric acid in the Purex process) in theannular container (1). Each pair of adjacent blades (6) (see FIG. 3)forms an inclined slit nozzle (7). The gradient “α” of the spiral isbetween 1 and 20 degrees, and the angle “β” between the plane of theblades and the horizontal surface is between 15 and 60 degrees.

The annular container (1) (see FIG. 1) communicates, in this casethrough an inclined conical slit (8) and the drain (4), with a pulsationchamber (9). In the illustrated embodiment the pulsation chamber iscylindrical and is situated coaxially with the annular container (1); itis equipped in its lower part with an outlet, normally a pipe connection(10), to empty the apparatus. The pulsation chamber (9) communicates, inthis case via a pipe connection (11), with a pneumatic pulsator (notshown in FIG. 1), and a plate damping device (12) or other dampingdevice is situated inside the chamber to ensure that liquid in thechamber moves without waves or splashing. In the annular gap between thecontainer (1) and the pulsation chamber (9) is a neutron absorber (13)to ensure nuclear safety while nuclear fuel leaching is going on. Theneutron absorber (13) is of course dispensed with in non-nuclearapplications.

Suitable fluid inlets and outlets are provided for treatment liquid andfor gas. Thus, the illustrated apparatus includes pipe connections tofeed in solution (14) and blow off gases (15) connected to the upperpart of the annular container (1). An inlet for the solid phase material(in this case pipe connection (16) to feed in pieces of fuel pin withnuclear fuel) is connected to the lower part of the annular container(1). A pipe connection (17) is connected to pipe connection (16) toremove solution from the leaching apparatus; alternatively anothersolution outlet may be provided. A discharge point is provided for thedischarge of fuel hulls from the top of the ramp (19); specifically, apipe connection (18) is connected to an upper part of the container (1)to remove the tubular fuel hulls after dissolution of the fuel

In one embodiment of the pulsation apparatus, the gradient of the spiralin an upper part of the annular container (1) is greater than in a lowerpart. Thus, for example, the gradient of the spiral in the lower andmiddle parts of the apparatus may be set at 2 degrees, and in the upperpart at 4 degrees. This enables the time spent by the pieces in theapparatus to be extended for fuel pin pieces in which, for one oranother reason, there has not been full leaching of the fuel. Since suchpieces are of greater mass, an increase in the gradient of the spiralleads to a slowing of their movement towards the zone of discharge fromthe apparatus.

In another embodiment, the blades (6) (see FIG. 6) are made in the formof a trapezium and are fastened by their smaller end on the inside wall(2) of the annular container (1). This enables the optimum angle of theguide surface of the helical ramp towards the axis of symmetry of theapparatus to be obtained to compensate for centrifugal forces on thefuel pin pieces when the pulsation jets act on them to be compensated.

In another version of the invention, the width of the blades (6) isbetween 3 and 5 times the distances between them. This enables flat jetsof the liquid phase to be formed with pulsation of liquid in the annularcontainer, to move the layer of fuel pin pieces upwards along the spiralguide ramp. Irrespective of the blade width, the distance between theblades (6) at the outside wall of the container is suitably 0.4-0.9times the external diameter of the fuel pin. This ratio of sizesprevents blockage of the slits by fuel pin pieces, and reduces thehydraulic resistance of the apparatus and pulsation energy losses.

The apparatus operates as follows. The annular container (1) and thepulsation chamber (9) are filled with fuel leaching solution, whichcomes in through the pipe connection (14). The feed of sheared fuel pinpieces, which takes place over a set time, is via the pipe connection(16) onto the lower section of the spiral ramp formed by the blades (6).Pneumatic pulses from the pulsator (not shown in the Figures), whichhave set parameters for gas pressure, frequency and shape ofoscillations along the pipe (11) (see FIG. 1), enter the pulsationchamber (9). Under the influence of these pulses, the solution fillingthe pulsation chamber (9) and the annular container (1) moves in anoscillating (reciprocal) fashion at a set frequency, amplitude andpulsation shape. The level damper (12) which is in the pulsation chamber(9) ensures that the solution moves without waves or splashing.

These oscillations are transferred via the slit channel (8) to thesolution in the annular container (1). Passing through the slit nozzles(7) formed by the blades (6) (see FIG. 3) of the spiral ramp, thesolution forms flat pulsation jets. When the solution in the annularcontainer (1) moves upwards under the influence of these jets, the fuelpin pieces are taken away from the surface of the spiral ramp formed bythe blades (6) and move along and up it by a certain distance. When thecompressed gas is released from the pulsation chamber (9) into theblow-off, the solution in the container (1) moves downwards through thestatic difference in levels in the annular container (1) and thepulsation chamber (9). This makes the solution press the fuel pin piecesto the blades (6) of the ramp, and they move no further along the rampuntil the next pulse. In subsequent pulsation cycles, the process isrepeated and the fuel pin pieces gradually move along the spiral ramp inan upward direction. Moving along the spiral ramp, the fuel pin piecesgradually rise upwards through the apparatus and at the end of the rampthey pass out of the apparatus via the pipe connection (18), along witha certain quantity of solution.

Fresh solution entering the annular container (1) via the pipeconnection (14), moves towards the fuel pin pieces, dissolving the fuelcontained in them, and leaves the apparatus through a pipe connection(17), which is connected to the loading channel (16). To remove gasesformed during the process of spent fuel dissolution, a blow-off pipe isprovided on the cover (5) of the annular container (1). As the fueldissolves in the fuel pin pieces, their mass diminishes, and their rateof movement along the ramp rises. To reduce the rate of movement of thefuel pin pieces in which fuel still remains, the design of the apparatusenvisages an increase in the gradient of the spiral ramp in the upperpart of the apparatus.

The invention in preferred embodiments provides pulsation apparatushaving the combination of spatial and geometric characteristics whichwill give the most effective hydrodynamic conditions for stable movementof pieces of sheared fuel pin up the spiral in the annular space withfuel pieces having a length to diameter ratio of between 1:1 and 6:1while preventing criticality in the apparatus and giving it a highoutput.

The method of the invention is typically performed in the reprocessingof nuclear fuel, the method further including reprocessing the dissolvedfuel to form a fissile material optionally in the form of a fuel pellet,a fuel pin or a fuel assembly.

In another embodiment of apparatus in accordance with the presentinvention, the welded together blades of the above described embodimentare replaced by a single plate having transverse elongate slots locatedtherein.

We claim:
 1. A nuclear fuel dissolution apparatus which comprises aperforated sloping ramp contained within a process chamber forcontaining solvent for the fuel, a pulsation member which in use createspulses in solvent in the process chamber, the perforations beingdesigned to direct pulses of solvent along and up the ramp, and adischarge point for fuel hulls disposed at an upper region of the rampcharacterized in that the ramp is made out of flat blades and theperforations of the ramp comprise inclined slits formed between theblades, wherein the ramp is spiral and the gradient of the spiral in anupper zone thereof is greater than in a lower zone.
 2. An apparatus ofclaim 1 in which the process chamber has an outer side wall which iscircular in cross section.
 3. An apparatus of claim 1 in which thegradient of the spiral is between 1 and 30 degrees.
 4. An apparatus ofclaim 3 in which the gradient is between 1 and 20 degrees.
 5. Anapparatus of claim 1 in which the angle between the plane of the bladesand the horizontal is between 10 and 60 degrees.
 6. An apparatus ofclaim 1 in which the inclined slits are no more than 10 fuel pindiameters in length.
 7. An apparatus of claim 1 in which the averagewidth of the blades is between 3 and 5 times the distance between them.8. An apparatus of claim 1 in which the distance between the blades atthe outside wall of the container is 0.4 to 0.8 times the fuel pindiameter.
 9. An apparatus of claim 1 in which the pulsation membercomprises a pulsation chamber disposed coaxially within the processchamber.
 10. A nuclear fuel dissolution apparatus which comprises aperforated sloping ramp contained within a process chamber forcontaining solvent for the fuel, a pulsation member which in use createspulses in solvent in the process chamber, the perforations beingdesigned to direct pulses of solvent along and up the ramp, and adischarge point for fuel hulls disposed at an upper region of the rampcharacterized in that the ramp is made out of flat blades and theperforations of the ramp comprise inclined slits formed between theblades, wherein the blades are made in the form of a trapezium and arefastened by the smaller end to a central blade support within theprocess chamber.
 11. A nuclear fuel dissolution apparatus whichcomprises a perforated sloping ramp contained within a process chamberfor containing solvent for the fuel, a pulsation member which in usecreates pulses in solvent in the process chamber, the perforations beingdesigned to direct pulses of solvent along and up the ramp, and adischarge point for fuel hulls disposed at an upper region of the rampcharacterized in that the ramp is made out of flat blades and theperforations of the ramp comprise inclined slits formed between theblades, wherein the pulsation member comprises a pulsation chamberdisposed centrally within the process chamber.
 12. An apparatus of claim11 in which a neutron absorber is arranged between the pulsation chamberand an inside wall of the annular container.
 13. An apparatus for thetreatment of solid articles by liquid, comprising a container having anouter side wall of circular cross section, a spiral ramp located in thecontainer, and a pulsator communicating with a lower part of thecontainer, and also pipe connections for feeding in and removing piecesof fuel pin, solution and gas, characterized in that the ramp is made upof flat blades placed one after another alone the spiral and formingbetween one another inclined slit nozzles and the perforations of theramp comprise inclined slits between the blades, wherein the gradient ofthe spiral in an upper zone thereof is greater than in a lower zone. 14.An apparatus of claim 13 in which the gradient of the spiral is between1 and 30 degrees.
 15. An apparatus of claim 14 in which the gradient isbetween 1 and 20 degrees.
 16. An apparatus of claim 13 in which theangle between the plane of the blades and the horizontal plane isbetween 15 and 60 degrees.
 17. An apparatus of claim 13 in which theaverage width of the blades is between 3 and 5 times the distancesbetween them.
 18. An apparatus for the treatment of solid articles byliquid, comprising a container having an outer side wall of circularcross section, a spiral ramp located in the container, and a pulsatorcommunicating with a lower part of the container, and also pipeconnections for feeding in and removing pieces of fuel pin, solution andgas, characterized in that the ramp is made up of flat blades placed oneafter another alone the spiral and forming between one another inclinedslit nozzles and the perforations of the ramp comprise inclined slitsbetween the blades, wherein the blades are made in the form of atrapezium and are fastened by the smaller end to a central blade supportwithin the process chamber.
 19. A method comprising dissolving fuel inchopped nuclear fuel pins in a nuclear fuel dissolution apparatus whichcomprises a perforated sloping ramp contained within a process chamberfor containing solvent for the fuel, a pulsation member which in usecreates pulses in solvent in the process chamber, the perforations beingdesigned to direct pulses of solvent along and up the ramp, and adischarge point for fuel hulls disposed at an upper region of the rampcharacterized in that the ramp is made out of flat blades and theperforations of the ramp comprise inclined slits formed between theblades.
 20. A method according to claim 19 comprising loading solventinto the process chamber, loading fuel pin pieces onto a lower region ofthe ramp and creating solvent pulses to transport the fuel in pin piecesup the ramp to a discharge point when the cladding hulls are dischargedfrom the ramp.
 21. A method of claim 19 which is performed in thereprocessing of nuclear fuel, the method further including reprocessingthe dissolved fuel to form a fissile material optionally in the form ofa fuel pellet, a fuel pin or a fuel assembly.